This kind of permission splitting has been requested a lot and we're hard at work at looking into the possibilities in order to best handle situations like yours. Unfortunately I don't have an estimation on when these features will be ready.
Currently, if a user can view assets within Lansweeper, they can view all the assets in the database, regardless of the location they were scanned from.
While there are ways of blocking Lansweeper from scanning a remote domain (blocking the required connections). Do note that data from both scanning servers will be gathered in the database and view-able to everyone who can view assets in Lansweeper.
So while you can prevent you admins from scanning the other site's domain, it will not prevent them being able to see the asset data. If you truly want to split who can see which assets. You will have to get multiple licenses (a smaller license per site). Having a separate license per site means that you have a separate database per site.